[FIX] purchase_requisition: change active_id to tender_id in widget context

this is due to commit 6ed774de614a2977a8d186a14ef2cd45a76e7a2e and a37bad205b13cec4f19f8e84e28801278e3e36ca which removes the active_id from context if the target==curent which is the case of this widget, we therefore can not use active_id here.

fix #2385
Merge branch '8.0' of https://github.com/odoo/odoo into 8.0-win32